DUBAI/WASHINGTON, June 13 (Reuters) - The United States blamed Iran for attacks on two oil tankers in the Gulf of Oman on Thursday that drove up oil prices and raised concerns about a new U.S.-Iranian confrontation.

Iran "categorically rejects the U.S. unfounded claim with regard to 13 June oil tanker incidents and condemns it in the strongest possible terms," the Iranian mission to the United Nations said in a statement on Thursday evening.

It was not immediately clear what befell the Norwegian-owned Front Altair or the Japanese-owned Kokuka Courageous, which both experienced explosions, forcing crews to abandon ship and leave the vessels adrift in waters between Gulf Arab states and Iran.

One source said the blast on the Front Altair, which caught fire and sent a huge plume of smoke into the air, may have been caused by a magnetic mine.

The firm that chartered the Kokuka Courageous tanker said it was hit by a suspected torpedo, but a person with knowledge of the matter said torpedoes were not used.

An unexploded device, believed to be a limpet mine, was spotted on the side of the Japanese tanker, a U.S. official told Reuters, speaking on condition of anonymity. If confirmed, the next steps might be to deactivate or detonate the device.

Crude oil prices spiked more than 4% after the attacks near the entrance to the Strait of Hormuz, a crucial shipping artery for Saudi Arabia and other Gulf energy producers. Prices later settled about 2% higher.

The United States, which has accused Iran or its proxies of carrying out a May 12 attack on four tankers off the United Arab Emirates' coast as well as May 14 drone strikes on two Saudi oil-pumping stations, squarely blamed Iran for Thursday's attacks.

"It is the assessment of the United States government that the Islamic Republic of Iran is responsible for the attacks that occurred in the Gulf of Oman today," U.S. Secretary of State Mike Pompeo told reporters.

Pompeo did not provide explicit evidence to back up the U.S. assertion.

"This assessment is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication," Pompeo said.

U.S. and European security officials as well as regional analysts cautioned, however, against jumping to conclusions, leaving open the possibility that Iranian proxies, or someone else entirely, might have been responsible.

"There are lots of moving parts and 'facts' at the moment, so my only advice would be treat things with extra caution," said one security official, who spoke on condition of anonymity.

'SUSPICIOUS'

Tensions between Iran and the United States, along with its allies including Saudi Arabia, have risen since U.S. President Donald Trump pulled out of a deal last year between Iran and global powers that aimed to curb Tehran's nuclear ambitions in exchange for sanctions relief.

Iran has repeatedly warned it would block the Strait of Hormuz if it cannot sell its oil because of U.S. sanctions.

Tensions have increased further since Trump acted at the beginning of May to force Iran's oil customers to slash their imports to zero or face draconian U.S. financial sanctions. Iran's oil exports have dropped to around 400,000 barrels per day in May from 2.5 million bpd in April last year.

In May, the Trump administration also said it would send more troops to the Middle East, citing what it saw as a threat of potential attack by Iran.

Iranian Foreign Minister Mohammad Javad Zarif described the tanker explosions as "suspicious" on Twitter and called for regional dialogue. Tehran has denied responsibility for the May 12 attacks.

The crews of both ships struck on Thursday were picked up safely. The Bahrain-based U.S. Navy Fifth Fleet said it had assisted the two tankers after receiving distress calls.

U.N. Secretary-General Antonio Guterres told a meeting of the Security Council on cooperation between the United Nations and the League of Arab States that the world could not afford "a major confrontation in the Gulf region".

The Security Council discussed the attacks behind closed doors on Thursday at the request of the United States.

Kuwaits U.N. ambassador, Mansour Al-Otaibi, president of the council for June, said after the meeting that all 15 council members had condemned the attacks.

When asked if the United States had shown any evidence to support its accusation that Iran was responsible, Al-Otaibi told reporters: "We didnt discuss any evidence."

IRAN, U.S. SAY WAR SHOULD BE AVOIDED

Both Iran and the United States have said they want to avoid war.

"Iran will never initiate a war but will give a crushing response to any aggression," Iranian President Hassan Rouhani said on Wednesday.

U.S. Central Command said in a statement on Thursday evening that "we have no interest in engaging in a new conflict in the Middle East. We will defend our interests, but a war with Iran is not in our strategic interest, nor in the best interest of the international community."

Pompeo said U.S. policy remained making economic and diplomatic efforts to bring Iran back to negotiations on a broader deal.

The Iranian U.N. mission's statement said: "It is ironic that the U.S. who unlawfully withdrew from the Joint Comprehensive Plan of Action now calls Iran to come back to negotiations and diplomacy," using the formal name of the 2015 nuclear accord.

In abandoning the deal, Trump made clear he wanted Iran to curb not merely its nuclear work but its development of missiles and support for proxy forces in Iraq, Syria, Lebanon and Yemen.

Some regional analysts said they thought the attacks were likely to have been carried out by Iran and described them as a way for Tehran to try to acquire negotiating leverage and perhaps increase global pressure for U.S.-Iran talks.

"There is always the possibility that somebody is trying to blame the Iranians," said Jon Alterman of Washington's Center for Strategic and International Studies, referring to a so-called false flag operation to implicate another nation.

"But there is the greater likelihood that this represents an effort to bolster Iranian diplomacy by creating a perceived international urgency to have the United States and Iran talk," Alterman said.

By publicly blaming Iran, the administration may hope to get Tehran to stop such attacks, said U.S. foreign policy veteran Dennis Ross, now at the Washington Institute for Near East Policy.

"By attributing responsibility, by elevating and publicizing it, (they may hope) that the Iranians will realize they need to take a step back," said Ross, adding: "Im not sure that is going to be the result."

"The Iranians, in response to our maximum pressure, are practicing their own version of maximum pressure," he said.

Japanese Prime Minister Shinzo Abe was visiting Tehran when Thursday's attacks occurred, carrying a message for Iran from Trump. Abe, whose country was a big importer of Iranian oil until Trump ratcheted up sanctions, urged all sides not to let tensions increase.

Iran said it would not respond to Trump's overture, the substance of which was not made public.